With their sharp teeth, powerful jaws and muscular, heavily armored bodies, alligators and crocodiles rank among the most intimidating predators. But not all of the crocodilians' fearsome traits are so readily apparent. Part of what makes these animals such awesome hunters is their ability to operate under cover of darkness, often lurking half submerged, waiting for some unfortunate land creature to disrupt the surface of the surrounding water. According to findings published today in the journal Nature, the key to the crocodilian's keen sensory perception lies in tiny pressure receptors that cover the face much like a stubbly beard.

Behavioral studies conducted by Daphne Soares of the University of Maryland showed that these diminutive, dome-shaped sensory organs enable half-submerged alligators to orient themselves to a single water droplet in complete darkness without the use of hearing. In contrast, when she covered the dome pressure receptors with a plastic elastomer, the beasts failed to turn or lunge toward the subtle disturbance.

Close inspection of the receptors revealed that immediately above them the outermost layer of skin and the underlying keratin are 40 and 60 percent thinner, respectively, than in surrounding areas. In addition, each receptor is innervated by the trigeminal nerve--the same nerve that innervates electrosensory organs in the platypus and infrared detectors in snakes.

As to when in crocodilian evolution these extraordinary sensory organs arose, Soares found that only those extinct creatures that led semiaquatic lives (as opposed to terrestrial ones) showed the same telltale pattern of nerve canals associated with the receptors in extant forms. She thus posits that they emerged some 200 million years ago in the Early Jurassic period. "It's fun to imagine these enormous extinct crocodiles sitting halfway submerged in the water at night, waiting for dinosaurs to come and drink," Soares muses. "Just at the moment the dinosaur broke the water surface with its mouth, it would have sent pressure waves in the water, telling the crocodile where to get its next meal."